Silicon wafers when bonded together are used in a variety of applications. Bonded silicon wafers are used to produce many types of devices including pressure sensors, insulated gate bipolar transistors, CMOS devices, high voltage transistors, and thermal ink jet printheads. Any number of silicon wafers can be bonded together, however, a majority of applications require the bonding of two silicon wafers. A silicon wafer pair, if properly bonded, can produce, depending on wafer size, up to several hundred or more individual thermal ink jet printheads.
Silicon wafer pairs are typically bonded together by the application of heat, electricity, pressure or any combination thereof. The application of heat is typically required to bond wafer pairs which have an adhesive bond or a chemical bond. The heat causes a chemical reaction to take place between the wafer pair or causes an adhesive to melt so that the wafer pairs bond together. The application of pressure to the wafer pairs insures that a bond takes hold with sufficient contacting force between the surfaces.
Different methods of applying pressure have been tried with varying success. One simple method used is to mate and align the wafers and place a weight on top of the wafer pair. Such a method is not extremely accurate, but wafers do bond with this method, although the yield rate of individual devices suffers. Another method uses a mechanical plunger having a rigid flat surface which contacts the top surface of the wafer pair. The mechanical plunger, is either driven from the top by mechanical force or is pulled down by the application of a vacuum to the underside of the plunger which is attached to a vacuum seal.
Mechanical plungers, however, suffer from the fact that the pressure may not be applied equally throughout the surfaces of the wafers, thereby causing inconsistent bonding throughout the wafer pair. In addition, due to the strict tolerances required in the bonding process, it is quite difficult, if not impossible, to bond more than one assembly or wafer pair at a time with any degree of success using methods applying top pressure through a flat rigid surface.
In U.S. Pat. No. 4,535,721 to Yakura, a process for making a stacked high voltage rectifier of silicon wafers is described. Wafers are prepared with dopant material, stacked, and held in a compression jig which maintains a substantial pressure on the wafer stack while it is heated.
U.S. Pat. No. 4,818,323 to d'Aragona et al. describes a silicon wafer bonding technique utilizing low pressures and a dissolvable gas to substantially eliminate voids formed between the bonding surfaces of the two wafers. Wafers are joined and placed in a low or reduced pressure furnace and heated at a temperature of about 800 degrees centigrade for approximately one hour.
U.S. Pat. No. 4,953,287 to West et al. describes a thermal bonding apparatus comprising an air actuated ram with a flange having an O-ring seated therein to seal a pliable sheet against a vacuum chuck. The vacuum chuck holds a printhead assembly. Pressurized air is admitted into a chamber included in the ram to force the pliable sheet against the printhead assembly during heating to bond the printhead assembly.
U.S. Pat. No. 5,054,683 to Haisma et al. describes a method of bonding together two bodies, the first body having a flat surface and the second body coated with a silicon oxide layer and also having a flat surface. A connecting layer of boron is applied to one of the flat surfaces and the bodies are pressed against one another for a time at an elevated temperature.
U.S. Pat. No. 5,105,430 to Mundinger et al. describes a laser diode array and cooling means that are connected in a compact, thin planar assembly having the laser diode located proximate to one edge. The planar assemblies comprise three wafers. The wafers are aligned and bonded in a conventional alignment jig which contains vacuum holes positioned adjacent to corresponding holes in the center.
U.S. Pat. No. 5,131,968 to Wells et al. describes an apparatus and method for bonding wafers together. A first wafer is vacuum mounted on a flat chuck and a second wafer is mounted on a convex pressure gradient chuck. The chucks are moved together once the wafers have been scrubbed and polished. The convex pressure gradient chuck applies a pressure gradient to the wafer surfaces for bonding.